Minimally Invasive Physiologic Parameter Recorder and Introducer System

ABSTRACT

An implantable monitoring device includes a flexible lead body that includes at least one sensing element. The device also includes a rigid main body connected to the flexible lead body at an attachment point. The rigid main body is generally centered about a longitudinal axis defined by the flexible lead body when the lead body is unflexed. The device further includes a measurement circuit, which is housed within the rigid main body and electrically coupled to the at least one sensing element of the flexible lead body and at least another sensing element on an outside surface of the rigid main body. The measurement circuit is configured to measure a potential difference between the at least one sensing element of the flexible lead body and the at least another sensing element of the main body.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/362,812, filed on Jan. 30, 2009, and entitled “Minimally InvasivePhysiologic Parameter Recorder and introducer System” which claimspriority from U.S. Provisional Patent Application Ser. No. 61/024,875,filed on Jan. 30, 2008, and entitled “Injectable Physiologic ParameterRecorder,” and from U.S. Provisional Patent Application Ser. No.61/097,826, filed on Sep. 17, 2008, and entitled “Introducer System forMinimally Invasive Physiologic Parameter Recorder,” and herebyincorporates by reference the contents of each case in its entirety.

TECHNICAL FIELD

This disclosure relates to implantable monitoring devices, andimplantation systems and methods for implanting the monitoring devicesat a subcutaneous implant location.

BACKGROUND

Implantable devices that monitor cardiac physiologic activity arefrequently implanted subcutaneously under a patient's skin of the chest.An implantable loop recorder is an example of a device that may beimplanted in this fashion. The implanted device may be leadless or mayinclude subcutaneous leads. Two such devices are leadless and have arigid rectangular shape. Another device, the Sleuth, is shaped like asmall pacemaker, and includes a flexible lead extending from a header ofthe device. FIG. 1 shows an example of the Sleuth device. These devicescan be used to record an electrocardiogram (EGG) signal for the patient.

To implant the Sleuth device, a 25 mm incision is made, a subcutaneouspocket is formed near the incision, and a tunnel is formed to extendaway from the pocket for placement of the flexible lead using a tool orfinger. The device may be inserted through the incision and placed inthe subcutaneous pocket, tested for proper operation, and repositionedif necessary. The incision is then closed.

Implanting leaded devices in this manner may be difficult, especiallyfor physicians who are not skilled in device implantation. If the deviceis improperly implanted, undesirable complications for the patient orsuboptimal device performance may result. In addition, tearing of tissueduring formation of the pocket and tunnel, for example, may result intissue bleeding that requires appropriate steps during surgery to avoidhematoma. In addition, it may be necessary to employ fluoroscopy toassure that the flexible lead is properly positioned under the skin. Ifnot properly positioned, the lead may require repositioning to obtain anoptimal EGG signal. This may extend the surgery duration, which mayincrease risk of infection and trauma, as well as expense. A need existsfor an improved device shape and associated insertion system for asimpler approach to insertion, shorter insertion time, reduced risk ofcomplications, reduced expense, and a reduced need for expensiveequipment, such as fluoroscopy, during device placement.

SUMMARY

In a first general aspect, an implantable monitoring device includes aflexible lead body that includes at least one sensing element. Thedevice also includes a rigid main body connected to the flexible leadbody at an attachment point. The rigid main body is generally centeredabout a longitudinal axis defined by the flexible lead body when thelead body is unflexed. The device further includes a measurementcircuit, which is housed within the rigid main body and electricallycoupled to the at least one sensing element of the flexible lead bodyand at least another sensing element on an outside surface of the rigidmain body. The measurement circuit is configured to measure a potentialdifference between the at least one sensing element of the flexible leadbody and the at least another sensing element of the main body.

In various implementations, a portion of the rigid main body of thedevice may taper from a first width to a second width narrower than thefirst width. The portion may taper symmetrically about a longitudinalaxis of the rigid main body. The portion may taper approximatelylinearly from the first width to the second width, or may tapernon-linearly from the first width to the second width. A first portionof the rigid main body may taper approximately linearly from a firstwidth to a second width narrower than the first width, and a secondportion of the rigid main body may taper non-linearly from a third widthto a fourth width narrower than the third width. The rigid main body mayinclude at least a first housing section that is hermetically sealed anda second housing section that is not hermetically sealed. A width of aproximal section of the second housing section may be substantiallygreater than a width of a distal section of the second housing. Thedevice may also include loop member on a proximal portion of the rigidbody. The loop member may be used to suture the device to body tissue,and a withdrawal force may be applied to the loop member extract thedevice is extracted from an implant location.

In a second general aspect, an implantable monitoring device includes aflexible lead body. The device also includes a rigid main body connectedto the flexible lead body at an attachment point. The rigid main body isgenerally centered about a longitudinal axis defined by the flexiblelead body when the lead body is unflexed. The rigid main body includes atapered portion proximate the lead body, and the tapered portion hassmaller width nearer the lead body. The device further includes ameasurement circuit, housed within the rigid main body and electricallycoupled to at least one sense electrode on the flexible lead body and atleast another sense electrode on an outside surface of the rigid mainbody. The measurement circuit is configured to measure a potentialdifference between the at least one sense electrode on the flexible leadbody and the at least another sense electrode on the main body.

In a third general aspect, a method of implanting a monitoring devicesubcutaneously in a body of a patient includes assembling an introducer,which includes a sheath and a semi flexible insert, by placing thesemi-flexible insert within the sheath. The semi-flexible insert issized and shaped at least in part to match a size and shape of themonitoring device. The semi-flexible insert and the monitoring deviceeach include a tapered section that tapers from a first width at aproximal end of the section to a second width, smaller than the firstwidth, at a distal end of the section. The method also includesintroducing a distal end of the introducer through an incision in thepatient's skin to a desired subcutaneous implant location site beneaththe patient's skin. The method further includes withdrawing thesemi-flexible insert from the sheath without substantially disturbingthe position of the sheath at the desired subcutaneous implant locationsite, and inserting the monitoring device into the sheath. The methodfurther includes withdrawing, in a direction opposite that which it wasintroduced, the sheath from the implant location site while applyingpressure to the monitoring device, wherein an external surface of thesheath splits along an axis as the sheath surface is forced against thetapered section of the monitoring device while the sheath is beingwithdrawn.

In various implementations, at least a portion of the sheath may besized and shaped in proportion to a corresponding portion of thesemi-flexible insert. The distal end of the introducer may deflect uponcontacting a surface of a muscle layer and slide across the surface ofthe muscle layer without penetrating the muscle layer. The externalsurface of the sheath may include a surface modification along at leasta portion of the axis, and the surface modification may reduce a tensilestrength of the external surface of the sheath along the axis.

In a fourth general aspect, an introducer system for implanting, withina body of a patient, a monitoring device that includes a tapered sectionthat tapers from a first width at a proximal end of the section to asmaller second width at a distal end of the section, at a subcutaneousimplant location site within the body of the patient, includes asemi-flexible insert. At least a portion of the semi-flexible insert issubstantially sized and shaped to match a portion of the monitoringdevice, including the tapered section. The system also includes a sheathsized and shaped to separately receive, within a space defined by aninternal surface of the sheath, the semi-flexible insert and themonitoring device. The sheath is splittable along a longitudinal axis ofthe sheath to facilitate removal of the sheath from the subcutaneousimplant location site.

In various implementations, a distal portion of the sheath may be sizedand shaped in proportion to a corresponding distal portion of thesemi-flexible insert. The sheath may be more flexible than thesemi-flexible insert, and a distal portion of the semi-flexible insertmay have sufficient rigidity to avoid substantial deflection whendirected through a fatty tissue layer of the patient, and sufficientflexibility to, upon contacting a surface of a muscle layer of thepatient, deflect and slide across the surface of the muscle layerwithout penetrating the muscle layer. An external surface of the sheathmay include a surface modification along at least a portion of thelongitudinal axis, and the surface modification may reduce a tensilestrength of the external surface of the sheath along the longitudinalaxis. The system may also include a rod member preformed to define anarc angle, where the rod member may be more rigid than the semi-flexibleinsert, and where the semi-flexible insert may define a cavity capableof receiving the rod member.

In a fifth general aspect, a method of implanting an implantablemonitoring device—which includes a rigid main body and a flexibleextension that, when unflexed, is substantially collinear with alongitudinal axis of the rigid main body—in a subcutaneous implantregion of a patient includes introducing an insert device to thesubcutaneous region of the patient. The insert device has an internalchamber that is generally in the shape of at least a portion of theimplantable monitoring device. The method also includes inserting, afterthe insert device has been introduced to the subcutaneous region, theimplantable monitoring device to the internal chamber of the insertdevice. The method further includes removing the insert device from thesubcutaneous region while leaving the implantable monitoring device atthe subcutaneous region.

In various implementations, removing the insert device from thesubcutaneous region includes withdrawing, in a direction opposite thatwhich it was introduced, the insert device from the subcutaneous regionwhile applying pressure to the monitoring device, where the insertdevice includes a surface modification to reduce a tensile strength ofthe insert device. The subcutaneous region may be above a pectoralfascia of the patient. At least a portion of the subcutaneous region maybe below a pectoral fascia of the patient, or the entire subcutaneousregion may be below the pectoral fascia.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features,objects, and advantages will be apparent from the description anddrawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art physiologic signal monitoring device.

FIG. 2 is an elevation view of an exemplary implantable device that canbe subcutaneously implanted under a patient's skin.

FIG. 3 is an elevation view of the exemplary implantable device of FIG.2 with a curved lead orientation.

FIG. 4 is a top view of an exemplary implantable device showing selectedcomponents housed within the device.

FIG. 5 is an elevation view an exemplary introducer system that may beused to introduce the implantable devices of FIGS. 2-4 to a subcutaneousimplant site within a body.

FIG. 6 is an elevation view of an exemplary semi-flexible insert.

FIG. 7 is an elevation view of an exemplary sheath.

FIG. 8 is an elevation view of an exemplary implantable devicepositioned within an exemplary sheath.

FIGS. 9A and 9B are elevation views of exemplary semi flexible insertsthat include cavities.

FIG. 10 is an elevation view of various exemplary insertion pins.

FIG. 11 is an elevation view of a semi-flexible insert with a curveddistal portion.

FIG. 12 is an elevation view of a sheath and a semi-flexible insert withcurved distal portions.

FIG. 13 is a block diagram of circuitry that may be included inimplementations of the implantable device disclosed herein.

FIG. 14 is a diagram of an exemplary system.

FIG. 15 is a view of an introducer system being introduced through anincision and a fatty tissue layer.

FIG. 16 is a view of the introducer system of FIG. 15 after a distalportion of the system has contacted a muscle layer.

FIG. 17 is an elevation view of an exemplary implantable device.

FIG. 18 is an elevation view of another exemplary implantable devicethat can be subcutaneously implanted under a patient's skin.

FIG. 19 is an elevation view of a sheath and an implantable device withcurved distal portions.

FIG. 20 is a view of a device implanted subcutaneously in a patientusing the introducer system of FIGS. 15-16.

FIGS. 21-22 are exterior views of an exemplary implantable device thatincludes at least two rigid sections separated by at least one flexiblesection.

FIG. 23 is a view of the exemplary implantable device of FIGS. 21-22,showing selected components housed within the device.

FIG. 24 is a cross-sectional view of an exemplary implantable devicehaving a flattened cross-sectional shape.

FIGS. 25-26 are views of exemplary implantable devices showing selectedcomponents housed within the devices.

FIG. 27 is an exterior view of an exemplary implantable device thatincludes three electrodes.

FIG. 28 is a view of an exemplary implantable device with a flexiblesection that includes a sleeve of wire braid or mesh.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 2 is an elevation view of an exemplary implantable device 100 thatcan be subcutaneously implanted under a patient's skin. Prior toimplantation, an introducer system can be used to create and prepare animplant location site for the implantable device 100. Usingimplementations of the devices and techniques discussed herein, theimplantable device 100 may be implanted in a minimally invasive fashionthat minimizes an incision size for insertion, minimizes trauma to bodytissue during formation of an implant channel for the implantable device100, minimizes risk of puncture or intrusion upon a muscle layer,intercostal space or body organ, and provides a fitted implant locationclosely tailored to actual device dimensions. Because incision size maybe reduced as compared to previous implant techniques, a scar from theinsertion may be less noticeable. Also, by forming an appropriatelysized pocket for the implantable device 100, a risk of hematoma may bereduced. Further, the devices, systems and techniques disclosed hereinmay significantly reduce the time required for implantation, and maymitigate the need for fluoroscopy, thereby reducing the cost associatedwith the implantation procedure. Moreover, the simplicity of theapproach described here may make implantation feasible in a procedureroom or doctor's office, and may provide for consistently good resultswhen implanted by physicians who lack experience and skills in placingimplantable devices. For at least these reasons, physicians may preferthe systems, devices and techniques discussed herein when compared topresently available implant methods and devices.

By way of example, the device 100 may be a minimally invasiveimplantable monitoring device that senses and records a physiologicparameter, such as electrical activity of the heart, within a body of apatient. In some implementations, the device 100 is an implantablemonitoring device that senses and records a physiologic parameter, suchas an electrocardiogram (ECG) signal, within the body of the patient andwirelessly transmits information associated with the physiologicparameter to an external device. Such a monitoring-only device thatrecords cardiac electrical information may be implanted in a humanpatient for a relatively short period of time, such as a few months forexample.

Other physiologic parameters or combinations of parameters, such asother electrical signals (e.g., EEG signal, EMG signal, neural signal,bio-impedance signal), mechanical signals (e.g., blood pressure signal,blood flow signal), chemical signals (e.g., glucose), temperature andthe like may similarly be recorded by the device 100 in variousimplementations. The description that follows will focus withoutlimitation on implementations where the device 100 is used to monitor asubcutaneous ECG signal, but in other implementations such monitoringcould be combined with or substituted by other monitoring functions,such as arterial blood pressure monitoring, for example.

In some implementations, the device 100 may be relatively small, and maybe sized and shaped for convenient implantation within a body of apatient, such as at a subcutaneous implant site, for example, in apectoral region of a human patient, as will be discussed in more detailbelow. As can be seen with reference to FIG. 2, the device 100 hasgenerally a flat shape, and includes a distal end 102 and a proximal end104. FIG. 17 is another elevation view of the implantable device 100,and illustrates the generally flat shape of the device 100. Using anintroducer system that will be described more fully below, the device100 may be introduced distal-end-first at an appropriate implant sitefollowing formation of a preparatory channel using the introducersystem. In other implementations, the device 100 may be directlyinserted to a subcutaneous implant location, without using theintroducer system.

The exemplary device 100 generally includes three sections: a proximalsection 106, a distal extension 108, and a midsection 110 between theproximal section 106 and the distal extension 108. In variousimplementations, the proximal section 106 may comprise a hermetichousing, the midsection 110 may comprise a non-hermetic housing, and thedistal extension 108 may comprise a flexible lead.

The device 100 may include one or more electrodes for electricallyinterfacing to surrounding tissue for the purpose of sensing electricalactivity. In some implementations, device 100 includes two electrodes.For example, FIG. 18 shows an implantable device having a proximalelectrode 112 and a distal electrode 116, and may measure a potentialdifference (e.g., a subcutaneous ECG signal) between the proximal anddistal electrodes 112, 116. The electrodes 112 and 116 are each near alongitudinal end of the device 100. This placement may maximize signalvector length of a measured physiologic signal. In general, measuredamplitude of a sensed physiologic signal, such as an ECG signal, willvary with device placement and orientation within the patient. Sensedsignal amplitude may also be related to separation distance between themeasuring electrodes. Positioning the electrodes 112, 116 near oppositeends (e.g., near opposite longitudinal ends) of the device 10 maymaximize the amplitude of the sensed physiologic signal for a givendevice length, which may lead to better measurement results.

In other implementations, device 100 includes three electrodes, thoughany suitable number (one, two, three, four, five, etc.) may be used inother implementations.

Referring again to FIG. 2, a proximal electrode 112 comprises anexterior surface of the proximal section 106. A midsection electrode 114is located on an exterior surface of the midsection 110, and is acircumferential or ring electrode in this implementation. A distalelectrode 116 comprises a tip electrode in the depicted implementation,and is positioned near the distal end 102 of the distal extension 108.The device may measure a potential difference between any two of theelectrodes 112, 114, 116. For example, the device 100 may measure apotential difference between the proximal electrode 112 and the distalelectrode 116, between the proximal electrode 112 and the midsectionelectrode 114, or between the midsection electrode 114 and the distalelectrode 116. In an implementation, the device senses two vectors: afirst vector between electrode 112 and electrode 116, and a secondvector between electrode 114 and electrode 1 16. In someimplementations, one or more of the electrodes may comprise excitationelectrodes or combination excitation/sense electrodes. As an example,the device may measure a bio-impedance for diagnostic purposes byinjecting a known current between two electrodes and measuring aresulting voltage between two electrodes. In these implementations, thedevice 100 may similarly inject a current between any two of theelectrodes. The electrodes may comprise a conductive material such astitanium in some implementations, and any appropriate style of electrodemay be used for any of the electrode locations. In variousimplementations, the subsequent voltage measurement may occur across thetwo electrodes used to inject the current, across one electrode used toinject current and another electrode that was not involved with thecurrent injection, or across two electrodes different from the currentinjection electrodes.

A first end of the flexible lead 108 is attached at a fixation point tothe midsection 110, and may generally flex or bend about the fixationpoint, according to some implementations. The midsection 110 thusstabilizes the flexible lead 108, yet allows it to bend and flex aboutthe fixation point to conform to body tissue channel formation andsubsequent tissue movement and flexing as the patient's muscles contractand expand during daily activities. For example, with someimplementations it may be desirable to implant the device 100 such thatthe lead 108 extends collinearly with the proximal section 106 andmidsection 110, as shown in FIG. 2. In other implementations, it may bedesirable to implant the device 100 such that the lead 108 is flexed. Ingeneral, the flexible lead 108 may bend at any appropriate angle withrespect to the fixation point, and in any appropriate direction. Forexample, with reference again to FIG. 17 and the relatively flat profilepresented by the rigid body portion of the device, the lead may bend ina direction above or below a plane defined by a longitudinal axis of therigid body, and may do so at any appropriate angle with respect to theplane. For example, the lead may assume a curved orientation, as will bedescribed below, so that an appropriate vector angle between anelectrode on the lead and an electrode on the body of the device may berealized with respect to a plane or vector defined by the device, oranother measurement vector, according to some implementations.

Referring again to FIG. 2, the device 100 may generally be provided witha substantially flat cross-section to achieve good biocompatibility andpatient comfort, and so proximal section 106 may readily accommodateelectronic circuits and components that have flat construction, such ascircuit boards and components or integrated circuits that attachthereto, including certain batteries, for example. Such a shape, incombination with the improved implant site location formation methodsdiscussed herein, may also be conducive to physiologic measurements, inthat the device may be less likely to rotate about its longitudinalaxis. For example, device migration or rotation may be minimized, whichmay permit better measurement results as electrode surfaces may moveless with respect to contacting tissue. As such, a preferred orientationestablished at implantation time may be maintained. Maintaining desireddevice location and orientation at the implant site location may alsoimprove telemetry performance and power reception performance inimplementations utilizing a rechargeable battery, as challengesassociated with uncontrolled orientation or misalignment may be avoidedor minimized.

FIG. 3 is an elevation view of the exemplary implantable device of FIG.2 with a flexed or curved lead orientation. Using the techniques,systems, and devices described here, the device 100 may be implantedwithin a body such that the lead 108 has a curved orientation. As shown,the lead 108 has a radius of curvature “R,” and may conform tosurrounding tissue at the implant location. Curved lead orientationswith non-circular arcs may also be used, and implantation pockets forany appropriate lead arc angle may be formed at an implant locationsite, as will be described more fully below. With such an orientation,non-collinear measurement vectors may be realized between the deviceelectrodes 112, 114, and 116. For example, a first measurement vector“A” may be defined between the proximal electrode 112 and the midsectionelectrode 114, and a second measurement vector “B” may be definedbetween the proximal electrode 112 and the distal electrode 116. In someimplementations, an angle 118 of thirty degrees or more may separate themeasurement vectors A and B. Similarly, a third measurement vector “C”may be defined between the midsection electrode 114 and the distalelectrode 116. In implementations where the device 100 is implanted tohave two or more non-collinear ECG measurement vectors, improvedmeasurement performance may result. For example, multiple vectors can beuseful in assessing ECG morphology and for noise reduction. As such,multiple-vector measurement of subcutaneous ECG may provide a moreglobal assessment of cardiac condition. Also, depending on deviceorientation and body tissue interface characteristics, in some cases oneof the measurement vectors may provide weak signal reception, whileanother measurement vector may provide comparatively strong signalreception. In this case, having multiple measurement vector alternativesprovides redundancy, which may make the measurement system more robust.As will be described more fully below, an appropriate measurement anglebetween any two measurement vectors may be selected based on a suitableradius of curvature “R” or arc angle for lead 108, which may be variedby appropriate selection of a curved insert pin as part of an introducersystem implantation process.

Referring again to FIG. 2, the implantable device 100 includes a firsttapered section 120 and a second tapered section 122. The taperedsections 120, 122 may facilitate ease of insertion under the skin, andmay promote, in combination with the introducer system to be describedbelow, a snug-fitting implant site location for the device 100. Thefirst tapered section 120, generally corresponding to the midsection 110in this implementation, widens linearly from a first width 124 at thedistal-end side of the section to a larger second width 126 at theproximal-end side of the section for conversely tapers from the secondwidth 126 to the first width 124). The second tapered portion 122 alsowidens from a distal-facing first width 128 to a proximal-facing largersecond width 130 for conversely tapers in the opposite direction), butdoes so non-linearly, in arcuate fashion. In this example, width 126 isapproximately equal to width 128.

FIG. 4 is a top view of an exemplary implantable device 200 showingselected components housed within the device. Like the implantabledevice 100 of FIG. 2, device 200 includes proximal section 106 andmidsection 110, but includes a different style distal extension 202 inthis implementation, though the lead 108 from the FIG. 2 implantabledevice 100 could alternatively be used. As described above, the proximalsection 106 may comprise an external surface formed of a hermeticmaterial, such as a metal or ceramic. The proximal section 106 may housea battery 204, which may be single-use or rechargeable in variousimplementations, and circuitry 206 (e.g., an electronics module) forperforming actions consistent with the device's intended purpose.Without limitation, examples of actions that may be performed with someimplementations of the device include measuring one or more physiologicsignals, storing the measured signal (s) in memory within the device100, processing collected data, wirelessly transmitting or receivinginformation to/from an external device, and others.

The midsection 110 may include a non-hermetic external surface, and maybe designed to enclose or embed components suited for housing in anon-conductive enclosure, such as components that communicate by fieldor wave properties that may otherwise be impeded by a conductivehousing. In this implementation, the midsection 110 houses an antenna208 for wirelessly transmitting data to an external device or wirelesslyreceiving data from an external device. In some implementations, thenon-hermetic section may be molded with a polymer such as urethane toavoid having a conductive housing that could attenuate telemetry signalstransmitted from, or received by, the telemetry antenna 208. In variousimplementations, the non-hermetic section 110 may be radiopaque. In someimplementations, the midsection 110 can include a charging coil (notshown in FIG. 4) that can be excited (e.g., with an external chargingcoil placed in proximity to the implant location) to recharge arechargeable battery of the device. Hermetic feedthroughs 210 may beprovided where electrical connections enter or exit the hermeticproximal section 106 from the non-hermetic midsection 110 to maintainhermeticity of the proximal section 106. In some cases, electricalconnections through the interconnecting element 210 may take the form ofa helix, which may provide a long flex life. The electrical connectionsmay optionally be coated with parylene or other material to provide asecondary form-fitting barrier that may isolate each conductor.Protection offered by this secondary coating may especially benefitconductors carrying DC voltage, as factors such as dendrite growth thatcan adversely affect reliability may be minimized or prevented.

The distal extension 202 may be a flexible subcutaneous lead attached tothe midsection 110 at one end. Like the lead 108 of the device 100 shownin FIG. 2, lead 202 may include one or more electrodes for measuringelectrical activity or stimulating body tissue. In some implementations,the distal extension 202 can serve as the telemetry antenna for thedevice 200, and in these cases the depicted antenna 208 may be omitted.In some implementations, the telemetry antenna function is incorporatedinto the distal extension (lead) 202 independent from any ECG sensinglead functionality.

In general, extraction of devices (e.g., devices 100 or 200) accordingto any of the implementations discussed herein may include forming asmall incision in the skin and grasping the device from a proximal endwith an appropriate tool, such that it can be separated from attachedtissue and removed. Some implementations include a feature on anexterior surface of the device to facilitate grasping of the device. Forexample, a retraction loop 212 near the proximal end of the device maybe grasped or hooked in this fashion for ease of retraction. That is, attime of explant, a physician may grasp the loop 212, for example with agrasping tool, and apply an extraction force. In some implementations,the loop may be used as a suture hook to secure the device to tissue ata subcutaneous implant location.

FIG. 5 is an elevation view of an introducer system 500 that may be usedto introduce any of the implantable devices discussed herein (e.g.,devices 100, 200 of FIGS. 2-4) to a subcutaneous implant site within abody. In various implementations, the introducer system 500 includes asheath 502 and a semi-flexible insert 504. The semi-flexible insert 504may be placed within the sheath 502 (e.g., within a cavity defined bythe sheath), as shown in FIG. 5, and a distal tip 506 of thesemi-flexible insert 504 may protrude slightly (e.g., 1-5 mm) from adistal end 508 of the sheath 502. In an implementation, the distal tip506 of the semi-flexible insert 504 may be substantially blunt. Thesheath 502 and semi-flexible insert 504 may be introduced through anincision in the patient's skin and through a layer of fatty tissue belowthe skin. The distal end 506 of the semi-flexible insert 504 mayinitially create a channel through the body tissue as the introducersystem 500 is introduced. Tapered portions 503 of the introducer systemfurther assist in channel creation, as will be described more fullybelow.

As can be seen with reference to FIG. 5, portions of both the sheath 502and the semi-flexible insert 504 have size and shape substantiallysimilar to the size and shape of the implantable devices 100, 200discussed above. The sheath 502 and semi-flexible insert 504 of theintroducer system 500 can be used to create an implant channel andprepare an implant site location for the implantable device (e.g.,device 100 or 200), where the implant site closely matches the size andshape of the implant device. Using the introducer system 500, thechannel and implant location site may be created in a minimally invasivefashion that minimizes tissue disturbance and tissue trauma.

FIG. 6 is an elevation view of an exemplary semi-flexible insert, suchas the semi-flexible insert 504 depicted in FIG. 5. The semi-flexibleinsert 504 includes an external surface 550, portions of which are sizedand shaped to generally match the size and shape of portions of theimplantable device 100. For example, the semi-flexible insert 504includes a distal portion 552 with generally the same size and shape asthe distal extension 108 of the implantable device 100; a mid-portion554 with generally the same size and shape as the midsection 110 of theimplantable device 100; and a proximal portion 556 that includes a lowerproximal portion 558 and an upper proximal portion 560. The lowerproximal portion 558 has generally the same size and shape as theproximal section 106 of device 100 from the distal-facing side of theproximal section 106 to the point of maximum width of proximal section106. A distal-facing surface 562 of the upper proximal portion 560 maydefine a depression 564 in some implementations. A physician may applypressure to the distal-facing surface 562 of the upper proximal portion560 (as by placing her thumb against the surface) and may direct theintroducer system 500 to a desired implant location site.

The semi-flexible insert 504 may comprise a semi-flexible, semi-rigidpolyethylene material in various implementations. In someimplementations, the semi-flexible insert 504 is sufficiently flexibleto deflect upon contacting a muscle layer during the introductionprocess. For example, as the introducer system 500 is inserted throughan incision in the skin and through a fatty tissue layer below the skin,when the distal tip 506 (see FIG. 5) of the semi-flexible insert 504contacts a muscle layer below the fatty tissue layer, which maygenerally be harder and more dense than the fatty tissue layer, thedistal portion 552 of the semi-flexible insert 504 may deflect so thatthe introducer system slides across the top of the muscle layer ratherthan penetrating into the muscle layer. That is, as the introducersystem 500 is urged through a fatty tissue layer of the body below theskin at an angle relative to the muscle layer, when the distal end ofthe system contacts the muscle layer, the system should deflect to slideacross the muscle layer, substantially parallel to the muscle layer,even though the proximal end of the introducer system may continue to beurged at the same angle relative to the muscle layer. This may increasepatient safety, as muscle layer or intercostal space puncture may beavoided even if the physician directs the introducer system at asteeper-than-appropriate angle with respect to the muscle layer. Thesemi-flexible insert 504 should nevertheless be rigid enough to providedirection and maintain its shape and orientation without substantialflexing as the introducer system 500 is introduced through the skinlayer and the fatty tissue layer.

FIG. 15 is a view of an introducer system 980 being introduced throughan incision 981 in a skin layer 982 and through a fatty tissue layer984. As described above, the introducer system 980 may include a sheath(e.g., sheath 502) and a semi-flexible insert (e.g., semi-flexibleinsert 504), each of which may be sized and shaped in part to match orconform to a size and shape of an implantable device. In this fashion,an implant site location may be created that closely matches the sizeand shape of the implant device, for a snug-fitting pocket with residualtissue compression pressure on the implanted device to minimize risk ofdevice migration and tissue bleeding.

As shown in FIG. 15, the distal end of the introducer system 980 isapproaching a muscle layer 986 as the system is urged through theincision 981, skin layer 982, and fatty tissue layer 984, as by aphysician. FIG. 16 is a view of the introducer system 980 of FIG. 15after a distal portion of the system 980 has contacted the muscle layer986. As shown in FIG. 16, the introducer system may deflect and slidealong a top surface 988 of the muscle layer 986 without penetrating themuscle layer 986. When the introducer system 908 has been introduced tothe desired implant site location, the semi-flexible insert may bewithdrawn from the sheath and removed from the body through the incision981, leaving the sheath in place at the implant site location. Next, theimplantable device 100 may be inserted into the sheath at the implantsite location, and the sheath may be withdrawn from the site, as will bediscussed below.

FIG. 20 is a view of the device 100 implanted subcutaneously in apatient using the introducer system 980 of FIGS. 15-16. The implantabledevice 100 is positioned at an implant site location created by theintroducer system 980 below the skin 982 within the layer of fattytissue 984, just above the muscle layer 986. The implant site locationsize and shape is closely matched to the size and shape of the implantdevice 100 because of the size and shape of the sheath and semi-flexibleinsert (an optionally an insertion pin, described below) that comprisethe introducer system 980.

As described above, the semi-flexible insert 504 includes a firsttapered section 566 and a second tapered section 568. Like theimplantable device, the tapered sections 566, 568 of the semi-flexibleinsert 504 include opposing lateral surfaces that are tapered. The firsttapered section 566 widens linearly from a first width 124 at thedistal-end side of the section 566 to a larger second width 126 at theproximal-end side of the section 566 (or conversely tapers from thesecond width 126 to the first width 124). The second tapered portion 568also widens from a distal-facing first width 128 to a proximal-facinglarger second width 130 (or conversely tapers in the oppositedirection), but does so non-linearly, in arcuate fashion. In thisexample, width 126 is approximately equal to width 128. As such, thetapered portions 566, 568 of the semi-flexible insert 504 are sized andshaped to substantially match tapered portions 120, 122 of theimplantable device 100, which may facilitate formation of a fittedimplant location closely tailored to actual device dimensions.

FIG. 7 is an elevation view of an exemplary sheath, such as the sheath502 depicted in FIG. 5. The sheath 502 includes a distal portion 580with shape that generally matches the size and shape of a correspondingportion or portions of the implantable devices 100, 200, andsemi-flexible insert 504, described above. The distal portion 580 of thesheath 502 is sized to receive the semi-flexible insert 504 within thesheath 502, as shown in FIG. 5, and to receive the implantable device100 within the sheath 502, as shown in FIG. 8. That is, a surface of thesheath may be sized and shaped to receive the insert 504 or the device100, as by defining a cavity that closely matches or approximates thesize and shape of a least a portion of the device 100 and/or the insert504. The sheath 502 also includes a proximal portion 582 that includes,in the depicted exemplary implementation, first and second finger loops584 that may be grasped and pulled for sheath removal followingplacement of the implantable device 100, as will be described more fullybelow.

In various implementations, the sheath 502 may be formed of a highdensity polyethylene (HDPE), or of poly-tetrafluoroethene (PTFE). Insome implementations, the sheath may be radiopaque. In general, thesheath 502 may be more flexible than the semi-flexible insert 504, suchthat when the semi-flexible insert is placed within the sheath 502 andcaused to deflect, the sheath 502 may similarly deflect. The sheath 502should nevertheless be rigid enough to hold its shape and avoid collapsewhen the semi-flexible insert 504 is removed following introduction ofthe introducer system 500 to an implant location site. That is, when thesemi-flexible insert 504 is withdrawn from within the sheath 502 andremoved from the body, leaving the sheath 502 at the implant sitelocation, the sheath should generally maintain its shape withoutcollapsing due to tissue pressure on an outside surface of the sheath.

With reference to FIG. 8, the sheath 502 may include a surfacemodification along an axis 600 that runs longitudinally from a distalend 601 of the sheath 502 to an area 604 of the sheath corresponding toa maximal cross-sectional width of the implantable device 100 when thedevice 100 is inserted within the sheath 502. Examples of such surfacemodifications can include perforations 606 or scoring of the sheathsurface along the axis 600, notches or slits 602 along the axis 600, orreduced material thickness along the axis such that the sheath hasreduced tensile strength along the axis 600. In some implementations, afirst slit 602 a may be positioned along the axis 600 at a position ofthe sheath surface corresponding to the first tapered section 120 of theimplantable device 100 when the implantable device 100 is insertedwithin the sheath 502. Similarly, a second slit 602 b may be positionedalong the axis 600 at a position of the sheath surface corresponding tothe second tapered section 122 of the implantable device 100 when theimplantable device 100 is inserted within the sheath 502.

The surface modifications may be designed to permit the sheath 502 tosplit along the axis 600. For example, the modification or modificationsmay permit the sheath to split from the distal end 601 of the sheath tothe area 604 when the finger loops 584 are pulled away from the implantsite location while holding the implantable device 100 in place at theimplant site location (as by applying pressure at the distal end 104 ofthe implantable device sufficient to prevent the device 100 from movingin response to pressure exerted on the device 100 by the sheath 502 asit is being withdrawn). When this occurs, the sheath 502 may be expectedto split along the axis 600 at portions of the sheath surfacecorresponding to the tapered sections 120, 122 of the implantable device100, as these are the portions where force may be concentrated on thesheath. Optionally, a second surface modification may be included alonga second axis 600 on the opposite side of the sheath 502. In thisfashion, force applied to the sheath at the tapered portions 120, 122 ofthe implantable device 100 as the sheath is withdrawn from the implantsite location may cause the sheath 502 to split along the axes 600 (onopposite sides of the sheath), which may allow the sheath 502 to bepulled around the implantable device 100 and withdrawn from the body,leaving the implantable device 100 at the implant site location. Tissuetrauma may be minimized during sheath withdrawal because the sheath 502may remain substantially flat against the implantable device 100 as itis withdrawn. In various implementations, a single type of surfacemodification or a combinations of two or more surface modifications canbe used. The modification or modifications may be along all or a portionof the axis or axes, which axis or axes may be positioned at anyappropriate location along the sheath, in various implementations.

FIGS. 9A and 9B are elevation views of exemplary semi-flexible inserts700, 702, respectively, that define cavities through a portion of theinserts. Semi-flexible insert 700 defines an isodiametric cavity 704from a proximal end 706 of the insert 700 to near a distal end 708 ofthe insert 700. Semi-flexible insert 702 defines a cavity 710 from aproximal end 712 of the insert 702 to near a distal end 714 of theinsert 702. The diameter of the cavity 710 may be constant for theportion of the cavity corresponding to the distal portion 716 of theinsert 702. The diameter of the cavity may widen to a larger diameter,as shown in FIG. 9B, for the portion of the cavity corresponding to themid- and proximal portions 718 of the insert 702.

FIG. 10 is an elevation view of various insertion pins 750 that may beinserted into the cavities 704 or 710 of the semi-flexible inserts 700and 702, respectively. The insertion pins 750 may have constant length,and may be preformed to a particular arc angle. An insertion pin havingan appropriate arc angle can be used with the introducer systemdiscussed herein to form an implant site location pocket having adesired lead orientation for the implantable device 100. As shown inFIG. 10, insertion pin 750 d has an arc angle “a.” As can be seen inFIG. 10, insertion pin 750 c defines a larger arc angle than does pin750 d, and insertion pin 750 h defines a larger arc angle than does pin750 c. Insertion pin 750 a is substantially straight, such that the arcangle that it defines is about 180 degrees. The insertion pins 750 maygenerally be more rigid than at least the distal portions of thesemi-flexible inserts 700, 702. As such, when an insertion pin 750 isinserted into a cavity of a semi-flexible insert and pushed down intothe distal portion of the insert, the distal portion of the insert mayflex to assume the same arc defined by the insertion pin 750. FIG. 11 isan elevation view of a semi-flexible insert 760 with a curved distalportion 762. The distal portion 762 may be curved, for example, becausean insertion pin defining an arc (such as one of the pins 750) isinserted into a cavity defined by the insert 760, as described above.

In various implementations, the flexible insert 760 with the curveddistal portion 762 may then be inserted into a sheath (e.g., sheath502), which may cause a distal portion of the sheath to assume thecurved shape of the distal portion of the insert, as shown in FIG. 12.This introducer system (here including the sheath, semi-flexible insert,and insertion pin), may then be introduced into the body of a patient toform an implant channel and implant site location. When the implantabledevice is then placed in the sheath following withdrawal of thesemi-flexible insert, the flexible lead of the implantable device maycurve at the same arc angle, as shown in FIG. 19. In this fashion, animplantable device may be implanted in a body with a curved leadorientation (for example, see FIG. 3). In various implementations, thesheath and semi-flexible insert may be partially introduced into thebody initially, and then an insertion pin may be inserted into thecavity defined by the semi-flexible insert, which may cause distalportions of the insert and sheath to assume an arced orientation asdescribed above. The sheath, semi-flexible insert, and insertion pin maythen be advanced further to finish forming the implantation channel andthe implantation site location with a curved lead orientation.

FIG. 13 is a block diagram of circuitry 900 that may be included inimplementations of the implantable device disclosed herein. In someimplementations, the circuitry 900 or a portion thereof may be includedin the electronics module 206 shown in FIG. 4. Components or modules maybe combined or separated as desired, and may be positioned in one ormore portions of the implanted device. A filtering module 905 mayreceive a sensed physiologic signal and appropriately filter the signalto remove unwanted noise or to pare the received signal to informationin a desired frequency range, or above or below a desired frequencythreshold. An amplification module 910 may amplify the received signalfor processing, and an analog-to-digital converter 915 may convert theanalog signal to a digital signal. The digital signal may be storeddirectly into memory 925, or may first be processed by a signalprocessing module 920. Signal processing module 920 may includefunctions to extract information from the measured signal, or tocompress the measured signal to reduce the volume of data to store andtransmit. Memory 925 may include both volatile and non-volatile memory,according to various implementations, and may additionally storeinstructions that can be executed to perform actions. A control module930 may provide overall device control, and may include one or moreprocessors that can execute instructions and in response performactions. A telemetry module 935 may be used, in conjunction with thetelemetry antenna, for communication with an external device. Chargereception/control circuitry 940 may optionally be used inimplementations that include a rechargeable battery to control receptionof charge energy over a charge reception apparatus and coordinaterecharging of the battery. A battery monitoring module 945 may provideone or more of controlling the charge current/voltage as appropriate forthe type of battery, providing data that can be transmitted to a chargerduring charging to control and terminate charge time, assess a state ofthe battery from charge to depletion via voltage, impedance,charge-counting or other means, provide data to communicate to anexternal device for feedback as to when to charge or if an early chargeis required. For simplicity, connections between the various modules arenot shown in FIG. 13.

FIG. 14 is a diagram of an exemplary system 950. The system 950 includesan implantable device 100 implanted in a body of a patient 955. Theimplantable device 100 may correspond to any of the implantable devicesdiscussed herein and may be implanted according to any of theintroduction techniques disclosed herein. When implanted, the device 100may collect biological data from the patient 955. A handheld computingdevice 960 may be programmed to communicate wirelessly (e.g., transmitor receive data via radio frequency telemetry) with the implantabledevice 100. In some implementations, an external charging device 965 maybe used to periodically recharge a battery of the implantable device100, though as discussed above the device 100 may alternatively use asingle-use battery in some implementations. In various implementations,the patient 955 may use the handheld device 960 to manually initiatedata collection by the implanted device 100 (e.g., initiate ECG signalsensing and recording). For example, if the patient 955 feelslightheaded or feels palpitations in her chest, she may press a button967 on the handheld device 960, and the handheld device 960 maywirelessly command the implanted device 100 to record and storephysiologic data. The implanted device 100 may also record a physiologicsignal when it determines that such recordation may provide usefulinformation. For example, the device may monitor a biological parameter(e.g., heart rate), and may record an ECG signal based on predeterminedcharacteristics of the biological parameter. In some implementations,the device 100 may periodically record sensed physiologic informationaccording to a predetermined schedule. For example, the device mayrecord a strip of data (e.g., covering a predetermined number of heartbeats or having a predetermined strip duration or length) once everyminute, every several minutes, every hour, every several hours, everyday, every several days, etc.

The implanted device 100 may periodically transmit collected data to thehandheld device 960, such as every few hours or once per day, forexample. In some implementations, the implantable device 100 maytransmit sensed data in real time to the handheld device 960, and thehandheld device 960 may store the data in internal memory or display thedata as a waveform or otherwise on a display screen 970 of the handhelddevice 960. In some implementations, functionality of the handhelddevice 960 and the charger 965 may be combined within a single device.

A base station 975 may communicate (e.g., wirelessly) with the handhelddevice 960 and/or the charger 965, and may receive data from (or senddata to) either device in various implementations. The base station 975may transmit data over a network 980 to a remote server device 985,where the data may be processed and analyzed (e.g., by a physician or ahealth care provider). In some implementations, data analysis may occurwithin the implanted device 100, the handheld device 960, the charger965, or the base station 970. Data analysis can include detection ofcardiac anomalies based on the collected data.

Referring again to FIG. 2 and FIG. 6, the distal extension 108 of theimplantable device 100 may be substantially more flexible than thedistal portion 552 of the semi-flexible insert 504. The flexibility ofthe lead 108 may permit the lead to flex with body tissue afterimplantation at the implant location as muscles contract and expand, forexample. As described above, when the semi-flexible insert 504 of theintroducer system is introduced through the layer of fatty tissue, thedistal portion 552 substantially maintains direction without significantdeflection through the fatty tissue, until it encounters the musclelayer. Were the implantable device 100 to instead be initially insertedwithin the sheath 502 and introduced to the fatty tissue layer prior tochannel formation with the sheath 502 and semi-flexible insert 504, theflexible lead 108 may bend or kink, and may fail to maintain directionthrough the fatty tissue because of the higher flexibility of the lead108.

A method of inserting an implantable monitoring device—which may includea rigid main body and a flexible extension collinear with a longitudinalaxis of the rigid main body when unflexed—to a subcutaneous implantregion of a patient may include introducing an insert device having aninternal chamber that is generally in the shape of the implantablemonitoring device to the subcutaneous region of the patient. The methodmay also include introducing the implantable monitoring device into theinsert device, and removing the insert device while leaving theimplantable monitoring device at the subcutaneous region. For example,the sheath 502 may be considered an insert device that includes an innerchamber generally in the shape of the implantable device.

In various implementations, any of the implantable devices,semi-flexible inserts, or sheaths, may include a single tapered section,rather than two tapered sections. In some implementations, taperedsections may be omitted.

Some implementations of the sheath 502 may include one or morelow-profile surface features on or near the distal end of the sheath toprevent the sheath 502 from moving from the implant site location whenthe semi-flexible insert 504 is withdrawn from the sheath 502. As oneexample, one or more small protrusions may provide a bit of frictionwhen pulling backward on the sheath 502 (but not when pushing forwardduring introduction of the sheath and semi-flexible insert). Theprotrusion may be a low-profile shape having a raised portion facing theproximal end of the sheath in some implementations. For example, theprotrusion may have a triangular shape, with a longest edge of thetriangular shape (e.g., a hypotenuse if the triangle is a righttriangle) facing the distal end of the sheath. In some examples, acorner of the triangular shape with a largest angle between adjacentsides of the triangle may be the raised portion, or the portion of theshape most-raised with respect to the sheath surface. Other shapes (e.g.diamond shape or arrowhead shape) can also be used. The surface featureor features may provide enough friction to make it easy to extract thesemi-flexible insert 504 without disturbing the position of the sheath502 at the implant site location, but not enough to cause tissueabrasion when extracting the sheath after placement of the implantabledevice. In some implementations, the surface feature(s) may be omitted.While withdrawing the insert 504, the physician may apply a force to thesheath 502 to prevent the sheath 502 from moving as the semi-flexibleinsert 504 is withdrawn, for example.

Exemplary widths of the implantable devices discussed herein may be, attheir widest point, about 17.8 mm in one implementation and about 22.1mm in an alternative implementation, though even smaller widths arepossible. With devices having these widths, skin incisions as narrow as13.5 mm or 17 mm may be possible.

In some implementations, the implantable device 100 may besubcutaneously implanted without using the introducer system. Forexample, a physician may use a hemostat tool to grab, for example, thedistal tip of the flexible extension 108, and use the hemostat to insertthe device under the skin. The one or more tapered sections of theimplant device 100 may facilitate the insertion, in this example.

In an implementation, the device is implanted such that the rigid bodyof the device occupies an upper-pectoral-region subcutaneous position,and the flexible lead is routed inferiorly and located over a rightatrium of the patient. For example, a distal end of the lead may bepositioned near a middle of the lower chest, over the sternum.

FIGS. 21-22 are exterior views of an alternative exemplary implantabledevice 1010 that includes at least two rigid sections 1015 separated byat least one flexible section 1020. As shown in FIG. 22, the flexiblesection 1020 may flex or bend at an appropriate angle, which may permitthe implantable device 1010 to better fit a contour of a body followingimplantation in the body of a patient, as the device 1010 may conform tobody tissue at the implant location and flex with movement of thesurrounding tissue. This may provide increased comfort for the patient,and may permit improved functionality of the device, as will beexplained more fully below. The device 1010 may include thefunctionality of the implantable devices discussed above, according tosome implementations.

In some implementations, the device 1010 may be relatively small, andmay be sized and shaped for convenient implantation within a body of apatient, such as at a subcutaneous implant site, for example, in apectoral region of a human patient. For example, the device 1010 may begenerally cylindrical in shape (having a round cross-section) and mayhave a length within the range of about 4 cm to about 10 cm, an outsidediameter in the range of about 4 mm to about 10 mm, and may be insertedor injected under the skin of a patient using a trocar or similarinsertion device, according to some implementations. A roundcross-sectional shape may provide compatibility with existing round-boretrocars or other surgical tools. This implant method may result in onlya small, minimum-trauma entry point in the skin and a tunnel for thedevice, which may facilitate clean tissue healing that results in a lessnoticeable scar at the implant site as compared to conventionalimplantation procedures for larger devices.

FIG. 21 shows one example of an implantable device that has an overallshape that is elongate and generally tubular, with the longitudinal endsrounded. The example implantable device of FIG. 21 is made up of twoin-line, rigid, elongate tubular sections. A flexible portion connectsthe two rigid, elongate tubular sections, and allows the rigid sectionsto move (for example, bend) relative to one another. In addition, thedevice includes two electrodes, namely, a first electrode on a firstlongitudinal end of the device, and a second electrode on a secondlongitudinal end of the device. The electrodes are rounded.

As shown in FIG. 21, the device 1010 is in a substantially straightorientation, where the flexible section 1020 is not bent at an angle.The device 1010 may be placed in the trocar with this orientation forinjection into the patient. Upon delivery to the implant location, theflexible section 1020 may bend to conform to surrounding tissue at theimplant location, and thereafter may flex in response to patientmovement throughout the period of implantation. In variousimplementations, the flexible section 1020 may flex or bend in thelongitudinal direction, in the transverse direction, or in both thelongitudinal and the transverse directions. As an alternativeimplantation method, a physician may make a small insertion in thepatient's skin, and may form a subcutaneous tunnel to receive thedevice.

In the example depicted in FIGS. 21-22, electrodes 1025 are positionedat the longitudinal ends of the device 1010. The electrodes 1025 in theexample implementation are hemispherical in shape and are atlongitudinal ends of the device, although they may take any appropriateshape (e.g., button, ring, etc.) and may be placed at any appropriatelocation on the device. The depicted example includes two electrodes1025 a, 1025 b, but other implementations can include more electrodes(e.g., 3, 4, 5, 6, etc.), which may be used in various implementationsas sense electrodes and/or as stimulation electrodes. The electrodes1025 a and 1025 b are on the body of the device 1010, and are each neara longitudinal end of the device 1010. This placement may maximizesignal vector length of a measured physiologic signal. The length of thedevice 1010 may be varied depending upon its construction and upon adesired EGG signal amplitude. For example, in implementations where thedevice 1010 monitors an ECG signal from a subcutaneous pectoral implantlocation, measured amplitude of a detected R-wave may be about 10 uV per1 cm of electrode separation distance. Positioning the electrodes 1025a, 1025 b near opposite ends of the device 1010 may maximize theamplitude of the sensed physiologic signal for a given device length,which may lead to better measurement results.

In some implementations, the rigid sections 1015 may be hermeticallysealed and the flexible sections 1020 may be non-hermetic. In otherimplementations, some of the rigid sections 1015 may be hermetic andothers may be non-hermetic. In various implementations, exteriorsurfaces of the hermetic sections may be fabricated from a metal orceramic, and exterior surfaces of the non-hermetic sections may befabricated from a polymer. In some implementations, the flexiblesections 1020 may be of reduced diameter compared to the rigid sections1015 for improved flexibility, as shown in FIGS. 21-23. In otherimplementations, the flexible sections 1020 may be of diameters similarto those of the rigid sections 1015, such that, at the end of its usefullife, the device may be more easily extracted from the patient.

An exterior surface of the rigid sections 1015 may be fabricated of ametal such as titanium, according to some implementations. An exteriorsurface or a portion of an exterior surface of a rigid section may beused as an electrode (e.g., as a sense or stimulation electrode). Rigidsegments not used as an electrode may be coated with an insulator (e.g.,parylene), which may avoid formation of conductive paths across thedevice sensed parameter vector. If a portion of the rigid section 1015is to be used as an electrode, the remaining portion may be coated withan insulator.

The rigid sections 1015 may house electronics and components that permitthe implantable device 1010 to function. FIG. 23 shows the device 1010with a battery 1030, an electronics module 1035, a chargingapparatus/coil 1040 and a telemetry antenna 1045 disposed within therigid sections 1015 of the device 1010. In this implementation, thebattery 1030 and electronics module 1035 are grouped within the firstrigid section 1015 a, and the charging coil 1040 and the telemetryantenna 1045 are grouped within the second rigid section 1015 b. Section1015 a may be hermetic to protect the electronic components from watervapor ingress. Section 1015 b may be non-hermetic to allow the use ofenclosure materials that are non-conductive and will not attenuate theelectromagnetic signals transmitted or received by the enclosedcomponents. The flexible section 1020 separates the rigid sections 1015,and permits the device 1010 to flex and conform to surrounding tissue atthe implant location. This may permit the device to both stretch andflex to maintain high-quality electrode contact during body movement.

The rigid sections 1015 can be sized so that they are small enough toavoid causing skin irritation or discomfort for the patient, yet largeenough to hold the electronics or components that permit the implantabledevice to function. In some implementations, the rigid sections are lessthan about 3 cm in length. The sizes of various sections of the devicemay be varied with respect to other sections, whether by length, width,or shape. For example, rigid sections may be sized according to thecomponents or circuitry they will house.

FIG. 25 shows a device 1100 that has four rigid sections 1105. A firstrigid section 1105 a houses a battery 1110; a second rigid section 1105b houses one or more electronic modules 1115; a third rigid section 1105c houses a charge reception mechanism 1120, and a fourth rigid section1105 d houses a telemetry antenna or coil 1125. As shown in FIG. 25, therigid sections 1105 have varying sizes as appropriate. Flexible sections1130 separate adjacent rigid sections, and permit the device 1100 toflex to conform to body tissue at an implant location, in similarfashion as described above with respect to device 1010. The threeflexible sections (1130 a, 1130 b, 1130 c) may flex independently, eachbending an appropriate angle and an appropriate direction to conform toa tissue contour at the implant location. In various implementations,any number of rigid sections 1105 or flexible sections 1130 may be used.In implementations that include at least two flexible sections 1130, theflexible sections may bend such that the device assumes athree-dimensional orientation, rather than a planar orientation (e.g.,as with a single flexible section permitting bending in a singledirection), or a linear orientation (e.g., as with a rigid, straightdevice).

In various implementations, the components of the device can partitionedbetween rigid segments in a manner that minimizes the number ofinterconnects between segments. In one exemplary implementation (notshown), three rigid segments of length 2.5 cm house a battery,electronic circuitry for ECG measurement and telemetry, and a rechargingapparatus/coil and telemetry antennae, respectively. The segment withthe recharging apparatus/coil and telemetry antennae may be molded witha polymer such as urethane to avoid having a conductive housing thatcould attenuate the power or telemetry signals.

The flexible interconnect sections (1020, 1130) may take many differentforms. Material choices for the flexible sections may include elastomerssuch as silicone or polyurethane. The elastomer section may have asleeve of wire braid or mesh embedded, which may add strength or mayprohibit flexure beyond a predetermined angle. Since the elastomermaterials are non-hermetic, hermetic feedthroughs (not shown in thefigures) may be provided where electrical connections enter/exit thehermetic sections to maintain hermeticity of that section. Electricalconnections through the interconnecting element may take the form of ahelix, which may provide a long flex life. The electrical conductors maybe molded into the elastomer material to minimize or prevent voids wherewater may accumulate. The electrical connections may optionally becoated with parylene or other material to provide a secondaryformfitting barrier that isolates each conductor. In someimplementations, the helical conductors and elastomer may eachcontribute stiffness to the flexible section that, in combination, maypermit bending sufficient to move with body flexure but resist extremebending due to device migration or manipulation by the patient. Wherethere are adjacent hermetic segments, the hermetic feedthroughs for themating ends of the segments may be sourced as a single assembly,including contiguous conductors through and between them. This mayreduce cost and provide increased robustness for the device.

Referring again to FIG. 23, the device 1010 may use a rechargeablebattery 1030 and an associated charging mechanism 1040 to extendoperational life of the device 1010. Because the device has relativelysmall size, and because the device may be implanted in a patient for aperiod of several years, a rechargeable battery may provide a convenientalternative to extracting the device and replacing an expired battery.The battery 1030 can be recharged via magnetic field transmission ofenergy from outside the body. One or more coils 1040 in the device 1010may closely couple to one or more coils in a charging apparatus externalto the body. The external charging apparatus may take various forms, aswill be discussed later, but in general the external charging apparatusmay be positioned in relatively close proximity to the implanted devicelocation to facilitate efficient charging. Possible choices for thecharging frequency may include 125 KHz, 6.78 MHz (ISM), and 13.58 MHz(ISM), where ISM indicates frequency bands allowed by the FCC for use byIndustrial, Scientific, and Medical equipment that uses RF (radiofrequency) energy but not for the purpose of telecommunication. Energyat these frequencies may be largely transferred by magnetic field, asthe implant will be close enough to be in the “near-field” of thecharger. For patient convenience and to maintain close contact betweencharger and implant, the charger can be patient-worn and batterypowered. In some implementations, the external charger can beincorporated in a wearable clothing item or accessory that isunobtrusive yet holds the two devices in close alignment for optimalcoupling. The battery need not be rechargeable in some implementations,and in these cases the recharging coil 1040 may be omitted.

Recharging may alternatively be accomplished via ultrasound energy,light energy, or radio frequency energy originating from outside thebody. In an implementation, energy generated by movement (motion orflexure), chemicals, or temperature differentials within the body isused to recharge the battery 1030. Energy from body motion may beharvested within the device 1010 by components that convert motion toforce/displacement (e.g. via inertial forces on a weight), andforce/displacement to electrical energy (via a material such as apiezoelectric material), as will be discussed more fully below. Energyderived from body flexure (resulting in device flexure) may only requireconversion of force to electrical energy.

Device cross-sectional shapes other than circular may be used. FIG. 24shows a cross-sectional view of a device implementation having aflattened cross-sectional shape. The flattened cross section hasopposing relatively flat major sides 1060 and opposing rounded minorsides 1065. This implementation provides a flatter cross-section, whichmay readily accommodate electronic circuits and components that haveflat construction, such as circuit boards and components or integratedcircuits that attach thereto, for example. Such a shape may also be moreconducive to physiologic measurements, in that the device may be lesslikely to rotate about its longitudinal axis. The flatter shape, becauseit may minimize or prevent device rotation, may also permit bettermeasurement results as electrode surfaces may move less with respect tocontacting tissue. As such, a preferred orientation established atimplantation time may be maintained. Minimizing or preventing devicerotation and providing fixed and predictable orientation of a flat side1060 to the skin surface may provide an advantage where internaltelemetry or power reception coils have an axis perpendicular to thelong axis of the device. Such an orientation may help to overcomechallenges related to an implant that has migrated or rotated to createan uncontrolled orientation between the external charger and implanteddevice coils, as such misalignment may greatly decrease the rate atwhich the battery 1030 is charged.

A device with such a shape could be injected with a trocar that has adeformable bore or is preformed with a matching flattened bore. A trocarwith a round bore can be used to place an initially round but flexiblesheath that can then flex to insert the device with ovoid cross sectionthrough it after the tracer is removed. Alternatively, a tracer with around bore sized to accommodate the flattened cross section of thedevice could be used to inject the device without the sheath.

While harvesting energy from body flexure for powering electricalcircuitry or charging a battery may be used with any of theimplementations described herein, it may be particularly appropriate forimplementations that include multiple rigid hermetically sealed segmentsand flexible segments that interconnect them. When such body movementoccurs, forces from body flexure are applied to the rigid segments, andthen concentrated at the flexible interconnecting elements. One or morepiezoelectric elements may be incorporated within the flexiblesegment(s) (or within a flexible extension), which may flex in responseto the body movement and movement of the rigid segments relative to eachother. The one or more piezoelectric elements may then generateelectrical energy as a result of the movement. Such flexure may arise,for example, from a continuing repetitive body function such asrespiration, or intermittent voluntary flexure initiated by skeletalmuscles.

Some implementations include a liquid enclosed within one or moreflexible sections. The liquid can be used to accumulate the forceapplied to the flexible joint, as described above, and refer that forceto the piezoelectric element for generation of electrical energy thatmay be used to power electrical circuitry or charge a battery of thedevice. A space defined by the flexible section may be designed todeform with joint flexure such that volume and/or pressure of the liquidchanges in response to flexure of the joint. The piezoelectric elementcan be coupled to the space enclosing the liquid such that changes inliquid pressure deform the piezoelectric element and cause electricalenergy to be generated. In some implementations, the space may bedefined by a void within an elastomer such that extreme pressure changesare limited by compliance of the elastomer and do not rise to levelsthat would damage the piezoelectric element. In various implementations,the piezoelectric element may be mounted on a diaphragm to providestructural integrity and media isolation from the liquid. The liquid maybe chosen to have a chemical composition that minimizes permeationthrough the elastomer over time, and may be biocompatible so that anyfluid that is leaked to surrounding tissue has benign effect.Alternatively, the space containing the liquid may be defined by ametallic structure. The metallic structure may incorporate abellows-like feature to improve flex life and provide compliance tolimit pressure. This implementation may eliminate the need to choose theliquid based on permeability considerations.

FIG. 26 shows an implementation of a device 1200 that includes a rigidcenter section 1205 and two flexible sections 1210, each connected tothe rigid section 1205. In this implementation, the rigid section 1205is hermetic and houses a battery 1215 and electronics 1220 that controloperation of the device 1200. The flexible sections 1210 may berelatively short, flexible, stretchable extensions of the center section1205. First and second electrodes 1225 a and 1225 b (e.g., senseelectrodes or stimulation electrodes, or a combination) are attached,respectively, to the flexible sections 1210 a and 1210 b, and mayfacilitate high-quality electrode-tissue contact. The electrodes may becomposed of titanium in some implementations.

In some implementations, the flexible sections or extensions 1210 may becomprised of silicone. The flexible extensions 1210 may be designed toalso enclose or embed components suited for housing in a non-conductiveenclosure, such as components that communicate by field or waveproperties. In the depicted example, the leftmost extension 1210 aencloses a recharging coil 1230 and the rightmost extension 1210 bencloses a telemetry antenna 1235. An embedded recharging coil ortelemetry antenna may be designed with a wire-type and/or geometricconstruction that would allow the component to flex with the flexibleextension in response to a contour or movement of surrounding tissuematerial without sacrificing long-term reliability. Alternatively, thesecomponents 1230, 1235 may be of a rigid construction, which may allow awider selection of appropriate wire types and geometric, constructions.

A rigid component construction may be more resistant to long-termflexure failure, and may provide greater stability of electricalproperties of the component, such as resonant frequency. In the case ofrigid construction, the component 1230, 1235 may be enclosed or floatwithin a fluid-filled cavity defined by the extension 1210, which mayminimize or eliminate shear stresses associated with direct adhesion ofthe silicone to the component 1230, 1235. FIG. 26 shows a fluid 1240encapsulating the charging coil 1230 and the telemetry antenna 1235within cavities defined by their respective extensions 1210 a, 1210 b.Component encapsulation with fluid may also allow the silicone extension1210 to retain a higher degree of flexibility. The fluid used to fillthe cavity can be chosen to avoid loss by permeation through theflexible material, such as a perfluorocarbon for a silicone extension1210. Materials and geometries for conductive connections to theelectrical components (e.g., recharging coil, telemetry antenna, orsensing electrode) may be chosen to allow repeated flexure withoutfailure. In one implementation, an MP35-N Stainless material and ahelical geometry are used.

Another implementation includes, as an alternative to an electrode onthe body of the device (e.g., as shown in FIGS. 21-23 and 25) or on ashort flexible extension (e.g., as shown in FIG. 26), one or moreelectrodes incorporated on one or more longer, flexible leadsconstructed much like a short pacemaker lead. This approach may providefor a possibility of higher amplitudes of sensed ECG signals due to alonger sense vector, and may provide a more stable electrode placement.In some implementations, the lead or leads may be formed integral withthe body of the device, such that no connectors would be needed. Someimplementations include one electrode attached to one end of the body ofthe device, and a single lead with attached electrode extending from theopposite end of the device. Extraction of such a device may involvepulling the device from the lead, and in these implementations asufficiently high tensile strength of the lead may be specified. In someimplementations, the flexible ECG sensing lead also performs the role ofthe telemetry antenna through the use of the conductor connecting to theECG electrode or a separate conductor. In some implementations, thetelemetry antenna function is incorporated in a flexible leadindependent from any ECG sensing lead. In general, features of any ofthe implantable devices discussed herein may be combined or separated asappropriate.

FIG. 27 shows an exterior view of an exemplary implementation of adevice 1300 that includes three electrodes. First and second electrodes1305 are positioned on the body of the device at opposing ends of thedevice, in similar fashion to some of the embodiments discussed above.Additionally, a third electrode 1310, shown illustratively as acircumferential or ring electrode near the center of the device, isprovided. With three electrodes as shown, the device 1300 may measureany of three different physiologic signal vectors: a first vector 1315(labeled “A”) between the leftmost end electrode 1305 a and the middleelectrode 1310; a second vector 1320 (labeled “B”) between the rightmostend electrode 1305 b and the middle electrode 1310; and a third vector1325 (labeled “C”) between end electrodes 1305 a, 1305 b. The device1300 is curved to adapt to a tissue implant location, according to someimplementations. In some implementations, the device 1300 is a rigiddevice, while in other implementations one or more sections of thedevice 1300 may be flexible. Such a device may be inserted under theskin via a curved trocar or sheath, according to some implementations.The device 1300 may have a circular or flattened cross-sectional shape.The device 1300 may include more or fewer electrodes. Implementationsthat include additional electrodes would be able to measure additionalphysiologic signal vectors, which may provide a more global assessmentof the physiologic signal and associated body function. Any of theelectrodes 1305, 1310 may have any appropriate shape, including a buttonelectrode on a particular side of the device.

One implementation of a rigid device, such as the device 1300 shown inFIG. 27, may be implanted between ribs of a patient. Such a location mayprovide protection for the device and minimize tissue pinching. Forexample, an inframammary approach, such as disposing the device betweenribs 4 and 5 or between ribs 5 and 6, may provide a suitable locationfor sensing a high-quality ECG signal, and leave the device less exposedand less subject to movement or impact. Device reliability may beimproved as a result. The device may include any appropriate number ofelectrodes, and may be sized and formed appropriately for the targetpatient.

As described above, charging energy may be provided to the implantabledevice in a number of ways, and the device may include a component orcircuitry to interface with the external charging unit. For deviceimplementations that utilize magnetic-field recharging means, the devicemay include a coil (e.g., coil 1040, 1120, or 1230) for receiving chargeenergy that is as large as practical to maximize the energy transferrate. In some implementations, the coil is not be shielded by aconductive housing. At charge frequencies of about 125 KHz, however,attenuation through a conductive housing may be low enough to stillallow feasibility. In implementations where avoiding coil enclosurewithin a conductive housing is desirable, the coil may be incorporatedin one or more of the flexible segments. Alternatively, housingenclosing the charge reception coil may be ceramic, which can behermetic yet still pass the magnetic field energy with minimalattenuation. Implementations that include multiple flexible segments canhave coils in multiple segments to improve reception of charge energyand reduce charge times or increase a distance within which the externalcharge device must be maintained with respect to the implanted deviceduring charging.

Some implementations include multiple coils arranged in differentorientations within a single segment. For example, FIGS. 23 and 25 showcharge reception apparatuses 1040, 1120 having coils arranged inorthogonal orientations. When multiple coils are used, their output maybe summed, or the highest output coil may be selected via active orpassive means. Coils whose axis is aligned with the long axis of theimplant may be able to flex with the joint, as the helix formed by theturns of the coil may form a spring. Use of ferrite cores within coilsmay enhance coil ability to receive energy, but may reduce flexure ofthe coil and add weight to the device, and may create magnetic force onthe device during an MRI procedure. In another implementation, a rigidsegment contains a charge reception apparatus (e.g., one or more coils)and is molded with a polymer, such as urethane, with the polymer bodyforming the outline of the rigid segment.

Alignment of the charging coils (transmit coil in the charger andreceiving coil in the implant) can pose significant challenges in designof the charging apparatus, as it may be difficult to satisfy acombination of engineering and user constraints. In a simpleimplementation with lust one transmitting coil and one receiving coil,the coils may need to be kept in approximate alignment for optimalcoupling, preferably requiring minimal inconvenience and/or interactionfor the patient. Sources of misalignment that can occur over timeinclude migration of rotation of the implanted device, and inimplementations that utilize a body-worn charger, undesired movement ofthe charger. Misalignment can be minimized and its effects mitigated tosome extent with appropriate design of the coils, devices, andpatient-worn accessory that holds the charger, but some patientinteraction and compliance may still be required in someimplementations.

Some implementations provide feedback to the patient on energy transferrate and/or coil alignment. The implanted device may provide feedbackregarding power received and may transmit the feedback to the externaldevice, which may provide the feedback to the patient in any number ofways. The feedback to the patient may be audible, visual, tactile, orother used singly or in combination. In some implementations, thefeedback may be continuously provided, while in other implementationsfeedback may be provided only if power transfer dips below a predefinedlevel as a warning to inspect and correct the alignment. Someimplementations may provide patient feedback based solely on informationderived at the external charging device, such as the transmit coilloading, for example.

Some implementations that address coil alignment and power transfer usemultiple coils with different orientations or positions in the implanteddevice, the external charger, or both. As described above, charge orcharge reception apparatuses may include multiple coils havingorthogonal orientations to facilitate transfer of energy despite widealignment variations between transmitting and receiving apparatuses. Asone example, multiple coils may be used within the charger (or theimplanted device), and may be positioned side-by-side to create a largerarea for optimal coupling to the implant (charger). In variousimplementations, output of the multiple coils could be summed, or thehighest output coil could be selected via active or passive means. Apreferred charger coil may be determined by feedback communicated viatelemetry from the implant regarding power transfer as the chargerswitches between the different transmit coils, according to animplementation. Alternatively, multiple charger coils may be driven outof phase (e.g., 90.degree, out of phase) to create a rotating magneticfield that may be well-received by a single coil in the implant over avariety of orientations. As another alternative, a coil in the implanteddevice may be rotated on commutated connections, and may include a smallmagnet to align it with the charge coil in the external device. Such animplementation can include a magnet to provide the alignment force tothe implant coil/magnet.

As described above, the implanted device may provide feedback viatelemetry to an external charger. In other implementations, the variouscommunications and/or charge energy transfer may occur independently.Implementations that concurrently receive charge energy and transmitinformation may use energy directly from the charger for poweringtransfer of the ECG data. For this reason, it may be feasible to use asmaller battery with smaller charge capacity, and the useful life of thebattery may be extended. Also, the transfer of data from the implanteddevice to the external device may be performed at a reduced currentlevel, and may have improved reliability because the charger is alreadyproximal to the implant.

If the required data transmission intervals and charge intervals aresimilar (e.g. daily), then it may be reasonable to accomplish chargingand data transmission concurrently with the devices in close proximityto reduce energy needed for data transfer. Such a system may stillinclude provisions for a longer-range method of bidirectionalcommunication to accomplish a patient activation and transmittedresponse to activation. In some implementations where charging and datatransfer are performed concurrently, the two operations may be performedusing the same frequency or harmonics thereof, or they could beperformed at separate frequencies. Using separate frequencies may have aregulatory advantage in that, if an ISM frequency is used only forenergy transfer and not for communication, it may operate at a higherpower than if it is also used for communication. The power wouldpotentially be limited by the Specific Absorption Rate (tissue heating)and not by FCC limits. The communication from the implant for feedbackon power transfer and coil orientation could be performed at the samefrequency that is used for ECG data transfer, or at a differentfrequency.

Minimizing the time required to charge the implanted device andmaximizing the intervals between charging may improve patientcompliance. At the same time, and perhaps in conflict, it may be helpfulto align the charge cycle concurrent with (or reminded from) a recurringactivity that a typical patient already performs in a predictable cycle.The strongest human cycle is daily, with other cycles including weekly,semi-daily, etc. Another factor in determining an appropriate chargecycle is the cycle required to acquire (upload) data from the device,which may be approximately daily depending on the implant memory size,data compression, and the amount of source data.

Even with implementations where the external charger is body-worn, thecharge cycle may be more efficient if the patient is relatively inactiveduring charge cycles. Examples of appropriate times for charging theimplantable device may include during patient sleep, while the patientis seated during a meal, or any time the patient is seated, such as whenreading, watching TV, doing office work, or driving a vehicle.

As described above, the external charger may be included in a wearableaccessory. Alternatively, the charger may be positioned on the patient(e.g., positioned on the patient's chest while the patient is in asupine position) or adhered to existing clothing. As one example of awearable accessory that contains and positions the charger, a minimalvest may be provided with the overall positioning accomplished by holesfor the neck and at least one arm. Weight and thermal barrier propertiesof the vest and/charger may be minimized for patient comfort. In someimplementations, thermal isolation and/or cooling may be providedbetween the patient and the charger during charge cycles.

Many variations are possible. For example, the flexible sections may beformed using bellows to provide flexibility for the implanted device.Optionally, a sleeve may be provided over the bellows to prevent tissueingrowth into the convolutions of the bellows, which may make extractingthe device difficult. Referring again to FIG. 23, the device may beimplanted such that the end of the device near the telemetry antenna1045 is the trailing end, and has a shallower implant location versusthe end of the device near the battery 1030. This may facilitateimproved data telemetry and charge energy transfer.

Various implementations of the device may include interconnectingsegments, with at least one of the segments sufficiently hermetic forhousing electronic circuits for reliable operation when implantedchronically within a body of a human or animal. In some implementations,other segments of the device may not be substantially hermetic, but maybe at least somewhat flexible. Biopotential sense electrodes may bepositioned at or near ends of the device. Flexibility provided by thedevice may make the device more comfortable for the patient, and mayimprove the contact with tissue relative to a device body that is rigidalong its full length. The flexible segments may contain certainfunctional components of the device such as a communications antennae,recharging apparatus, and battery. The flexible segments may alsocontain interconnection to connect the sensing electrodes to the rigidsection(s) containing the signal processing electronics.

FIG. 28 is a cutaway view of an implementation of an implantable device1400 that includes two rigid sections 1402 separated by a flexiblesection 1404. The device 1400 includes rounded electrodes 1406 atopposing longitudinal ends of the device. The flexible section 1404includes a sleeve 1408 of wire braid or mesh, which may add strength tothe flexible section 1401 or may prohibit flexure beyond a predeterminedangle. For example, the sleeve 1408 may limit flexure of the flexiblesection to an angle 1410 (or less in some implementations), as shown.The example shows a particular angle 1410, but larger or smaller anglesare possible by modifying flexure properties of the sleeve 1408. In someimplementations, the sleeve may be embedded with the material comprisingthe flexible section, for example, or may alternatively be place overtop of the section. A battery 1412 and an electronics module 1414 areshown within one of the rigid sections 1402 b, and a telemetry antenna1416 and a charge reception apparatus 1418 are shown in the other rigidsection 1402 a. In implementations having multiple flexible sections,some or all of the flexible sections may include a sleeve similar tosleeve 1408, and in some cases sleeves having differing flexureproperties may be used on flexible sections (e.g., on different flexiblesections) of the same device. As such, device implementations with twoor more flexible sections where a first flexible section is limited to afirst angle of deflection and a second flexible section is limited to asecond angle of deflection, different from the first angle ofdeflection, are possible.

A layer of tissue, referred to as fascia, covers the pectoral muscle. Insome implementations, any of the devices discussed herein (e.g., devices100, 200, 1010, 1100, 1200, 1300, 1400) may be introduced to asub-fascial implant location. In some cases, introducing the device to asub-fascial location may reduce a risk of erosion and may provide a morestable implant location. In some implementations, the device may beimplanted such that the entire device remains above the pectoral fascia.In alternative implementations, the fascia may be penetrated and thedevice may be implanted such that the entire device is located below thepectoral fascia. In yet other implementations, the fascia may bepenetrated and the device may be implanted such that a distal portion ofthe device is positioned below the fascia and a proximal portion of thedevice is positioned above the fascia.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the techniques, devices, and systemsdiscussed herein.

1. A method of implanting a monitoring device subcutaneously in a bodyof a patient, the method comprising: a) assembling an introducer,comprising a sheath and a semi-flexible insert, by placing thesemi-flexible insert within the sheath, the semi-flexible insert sizedand shaped at least in part to match a size and shape of the monitoringdevice, the semi-flexible insert and the monitoring device eachincluding a tapered section that tapers from a first width at a proximalend of the section to a second width, smaller than the first width, at adistal end of the section; b) introducing a distal end of the introducerthrough an incision in the patient's skin to a desired subcutaneousimplant location site beneath the patient's skin; c) withdrawing thesemi-flexible insert from the sheath without substantially disturbingthe position of the sheath at the desired subcutaneous implant locationsite; d) inserting the monitoring device into the sheath; and e)withdrawing, in a direction opposite that which it was introduced, thesheath from the implant location site while applying pressure to themonitoring device, wherein an external surface of the sheath splitsalong an axis as the sheath surface is forced against the taperedsection of the monitoring device while the sheath is being withdrawn. 2.The method of claim 1 wherein at least a portion of the sheath is sizedand shaped in proportion to a corresponding portion of the semi-flexibleinsert.
 3. The method of claim 1 wherein the distal end of theintroducer deflects upon contacting a surface of a muscle layer andslides across the surface of the muscle layer without penetrating themuscle layer.
 4. The method of claim 1 wherein the external surface ofthe sheath includes a surface modification along at least a portion ofthe axis, the surface modification reducing a tensile strength of theexternal surface of the sheath along the axis.
 5. An introducer systemfor implanting, within a body of a patient, a monitoring device thatincludes a tapered section that tapers from a first width at a proximalend of the section to a smaller second width at a distal end of thesection, at a subcutaneous implant location site within the body of thepatient, the system comprising: a) a semi-flexible insert, at least aportion of which is substantially sized and shaped to match a portion ofthe monitoring device, including the tapered section; and b) a sheathsized and shaped to separately receive, within a space defined by aninternal surface of the sheath, the semi-flexible insert and themonitoring device, the sheath being splittable along a longitudinal axisof the sheath to facilitate removal of the sheath from the subcutaneousimplant location site.
 6. The introducer system of claim 5 wherein adistal portion of the sheath is sized and shaped in proportion to acorresponding distal portion of the semi-flexible insert.
 7. Theintroducer system of claim 5 wherein the sheath is more flexible thanthe semi-flexible insert, and wherein a distal portion of thesemi-flexible insert has sufficient rigidity to avoid substantialdeflection when directed through a fatty tissue layer of the patient,and sufficient flexibility to, upon contacting a surface of a musclelayer of the patient, deflect and slide across the surface of the musclelayer without penetrating the muscle layer.
 8. The introducer system ofclaim 5 wherein an external surface of the sheath includes a surfacemodification along at least a portion of the longitudinal axis, thesurface modification reducing a tensile strength of the external surfaceof the sheath along the longitudinal axis.
 9. The introducer system ofclaim 5 further comprising a rod member preformed to define an arcangle, the rod member being more rigid than the semi-flexible insert,and wherein the semi-flexible insert defines a cavity capable ofreceiving the rod member.
 10. A method of implanting an implantablemonitoring device—which includes a rigid main body and a flexibleextension that, when unflexed, is substantially collinear with alongitudinal axis of the rigid main body—in a subcutaneous implantregion of a patient, the method comprising: a) introducing an insertdevice to the subcutaneous region of the patient, the insert devicehaving an internal chamber that is generally in the shape of at least aportion of the implantable monitoring device b) inserting, after theinsert device has been introduced to the subcutaneous region, theimplantable monitoring device to the internal chamber of the insertdevice; and c) removing the insert device from the subcutaneous regionwhile leaving the implantable monitoring device at the subcutaneousregion.
 11. The method of claim 10 wherein removing the insert devicefrom the subcutaneous region comprises withdrawing, in a directionopposite that which it was introduced, the insert device from thesubcutaneous region while applying pressure to the monitoring device,wherein the insert device includes a surface modification to reduce atensile strength of the insert device.
 12. The method of claim 10wherein the subcutaneous region is above a pectoral fascia of thepatient.
 13. The method of claim 10 wherein at least a portion of thesubcutaneous region is below a pectoral fascia of the patient.
 14. Themethod of claim 13 wherein the entire subcutaneous region is below thepectoral fascia.